Many assays are now available which utilise the technology described in EP-A-291194 (equivalent to U.S. Pat. No. 5,622,871), wherein a particulate direct label such as a gold sol or coloured latex particle is used to reveal the result of an assay conducted in a porous carrier such as a porous strip. Concentration of the particulate label in a comparatively small detection zone in the strip reveals the assay result. It is common practice for the strip to include a control zone, normally located downstream from the detection zone, in which a coloured signal is also generated to reassure the user that the test has been correctly performed. In most commercial products based on this concept, the test and control signals are generated using the same particulate label.
This technology copes very well with most assay situations, but there can be extreme situations in which the clarity of the signals could be improved. For example, the control zone signal may appear rather faint if the concentration of analyte is very high and is causing most of the particle label to become bound in the detection zone, leaving insufficient label to carry through and provide a strong control signal.
An objective of the present invention is to provide reagents and assay devices in which improved clarity of assay signals are obtained irrespective of the amount of analyte that may be present in a sample being tested.
A further objective is to provide good clear assay signals without the use of excessive amounts of labelled reagent.